The petrochemical industry has long used hydrocarbon feedstocks for the production of valuable olefinic materials, such as ethylene and propylene. Ideally, commercial operations have been carried out using normally gaseous hydrocarbons such as ethane and propane as the feedstock. As the lighter hydrocarbons have been consumed and the availability of the lighter hydrocarbons has decreased, the industry has more recently been required to crack heavier hydrocarbons, such as naphthas and gas oils.
A typical process for the production of olefins from hydrocarbon feedstocks is the thermal cracking process. In this process, hydrocarbons undergo cracking at elevated temperatures to produce hydrocarbons containing from 1 to 4 carbon atoms, especially the corresponding olefins. Typically, the hydrocarbon to be cracked is delivered to a furnace comprised of both a convection and radiant heating zone. The hydrocarbon is initially preheated in the convection zone to a temperature below that at which significant reaction is initiated; and thereafter is delivered to the radiant zone where it is subjected to intense heat from radiant burners. Examples of conventional furnaces and processes are shown in U.S. Pat. No. 3,487,121 (Hallee), and U.S. Pat. No. 5,147,511 (Woebcke).
Illustratively, in the prior art, process fired heaters are used to provide the requisite heat for the reaction. The feedstock flows through a plurality of coils within the fired heater, the coils being arranged in a manner that enhances the heat transfer to the hydrocarbon flowing through the coils. The cracked effluent is then preferably quenched either directly or indirectly to terminate the reaction. In conventional coil pyrolysis, dilution steam is also employed to assist in reducing coke formation in the cracking coil.
In recent times, industry is requiring the building of larger plants which have increased capacity but which require less numbers of reactors. Thus, there has developed a need in the art to provide larger furnaces which are also flexible enough to handle a variety of different feedstocks to produce a variety of different olefin products. Because each different feedstock and desired product slate entails the use of different reaction conditions, primarily, reaction temperature and reaction residence time, none of the currently available furnace technologies are suitable. Previous attempts in the prior art to meet these increased capacity and flexibility requirements in a single furnace have proved insufficient.
It would therefore represent a notable advance in the state of the art if a furnace were developed which solved the problems of the prior art furnaces as described above.